A. Industrial Applications
The present invention relates to improvements in an apparatus for continuously measuring temperature of molten metal. More specifically, the invention relates to improvements in the apparatus for continuously measuring the temperature of molten steel in a tundish of a continuous casting process.
B. Prior Art
As an apparatus for continuously measuring the temperature of molten steel in a tundish by the use of a thermocouple, a publicly known arrangement is to insert a thermocouple into a protection tube whose compositions comprise alumina and graphite.
Since the temperature of the molten steel in the tundish is over 1500.degree. C., generally, as the thermocouple, a platinum-platinum/rhodium alloy thermocouple (including a type R thermocouple equipped with a positive electrode comprising an alloy of 87% platinum and 13% rhodium and a negative electrode comprising platinum, and a type B thermocouple equipped with a positive electrode comprising an alloy of 70% platinum and 30% rhodium and a negative electrode comprising an alloy of 94% platinum and 6% rhodium) is used.
It has been found that when the platinum-platinum/rhodium alloy thermocouple is used at high temperature, if it is exposed to a reductive atmosphere or metallic steam, carbon, sulfur, silicon and the like, it deteriorates rapidly, leading to burnout of the thermocouple.
For this reason, conventionally, inert gas is introduced into the alumina-graphite protection tube or a venting of the gas from the protection tube is carried out.
In addition, the conventional continuous temperature measuring apparatus has employed an arrangement in which the alumina-graphite protection tube, which is to be immersed in the molten steel in the tundish, is adapted to act as an outer tube, while a porcelain protection tube is adapted to act as an inner tube so that the thermocouple is inserted into the porcelain protection tube which is then inserted into the alumina-graphite protection tube. An alternative arrangement may be employed in which the porcelain protection tube has a triplex-layer construction, whose structure includes an inner sub-tube and an outer sub-tube. At the same time, a gap between the alumina-graphite protection tube and the porcelain protection tube is filled with electrically insulating powder, e.g. alumina powder.
The aforementioned prior art faces difficulties in replacement of the thermocouple, when such an accident as burnout of the thermocouple takes place in the apparatus, and in reuse of the apparatus, once it has already been used for a continuous measurement, while keeping the thermocouple housed in the porcelain protection tube.
In particular, when preheating the apparatus before immersing the same, a considerable temperature change takes place during the primary stage of such preheating which, along with other causes, has been found to give rise to burnout of the thermocouple, resulting in a demand for quick countermeasures against such a burnout, if necessary.